Sportster 1200, battery terminals

Rode about 90 min Thursday. Parked returned - no issues. Went to go for a ride today and it wouldn't start. It turned over a few times but wouldn't start. Took the battery out to put it on the charger, was getting about 12.6v on the bike and once connected to the charger. upon disconnecting I noticed the terminals both looked a lil melty. I have had to remove this battery and put it on the charger several times in the last year (between far and few occasional rides) so this isn't my first removal/charging rodeo but first time seeing this... What could cause this, and where should I start for troubleshooting?